Such a power module generally includes a power module substrate, in which a circuit layer is brazed on the front surface of a ceramic substrate while a metal layer is brazed on the rear surface of the ceramic substrate, a semiconductor chip soldered to the circuit layer, and a heat sink joined to the metal layer.
In order to improve the joint reliability of power modules, as disclosed in the Domestic re-publication of PCT International Publication for Patent Application No. WO 03/090277, for example, a circuit layer or a metal layer has conventionally been formed of an Al alloy having a purity of not less than 99.98 wt. % or pure Al.